Screenshot and title says it all, illusion blocks when given the grass block mimic only show the tinted texture without the base texture underneath, causing them to appear mostly invisible.

Sorry for not answering for two years. I completely forgot this issue. The illusion block is mostly useless for blocks with overlays. I recommend using the normal frame block and applying the grass overlay using the minecraft vanilla "grass" item. This has the same effect. But I know, this is not very intuitive, so I'll think of a solution for this...
Thanks for posting, regards, Manu